The lithium secondary battery has recently been widely used for example, as an electric source for driving small-sized electronics. The lithium secondary battery has a basic structure comprising a positive electriode, a negative electrode and a non-aqueous electrolytic solution, which are contained in a sealed cell. The positive electrode preferably comprises a complex oxide of lithium such as LiCoO2, and the negative electrode preferably comprises a carbon material or metallic lithium. A carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) has been advantageously used in the non-aqueous electrolytic solution for the lithium secondary battery.
The recent lithium secondary battery requires a further improvement on battery performance such as cycle characteristics of the battery and electric capacity.
In a lithium secondary battery, a complex oxide of lithium such as LiCoO2, LiMn2O4 and LiNiO2 is often used as a positive electrode material. A process of recharging the battery causes a local oxidation and decomposition reaction of a part of a solvent contained in a non-aqueous electrolytic solution. A decomposition product inhibits an ordinary electrochemical reaction of the battery to lower battery performance. The reason is considered that a solvent is electrochemically oxidized along an interface between the positive electrode material and the non-aqueous electrolytic solution.
In a lithium secondary battery, a highly crystallized carbon material such as natural or artificial graphite is often used as a negative electrode material. A process of recharging the battery causes a local reduction and decomposition reaction of a part of a solvent contained in a non-aqueous electrolytic solution. Ethylene carbonate (EC) is widely used as a solvent of the non-aqueous electrolytic solvent. Ethylene carbonate may particularly be reduced and decomposed to lower battery performance while repeating charge and discharge.
Japanese Patent Provisional Publication No. 8 (1996)-45545 and U.S. Pat. No. 5,626,981 recommend adding a vinylene carbonate compound to a non-aqueous electrolytic solution to improve battery performance of the lithium secondary battery. It is further reported that the cycle life is lengthened using the electric solution containing the vinylene carbonate compound.
Japanese Patent Provisional Publication Nos. 2000-195545, 2001-313072, 2002-100399 and 2002-124297 and U.S. Pat. No. 6,479,191 B1 recommend adding an alkyne compound to a non-aqueous electrolytic solution to improve battery performance of the lithium secondary battery. It is further reported that the cycle life is lengthened using the electric solution containing the alkyne compound.
Increase in density of a positive electrode composition layer or a negative electrode composition layer has recently been examined to enlarge capacity of the lithium secondary battery. Japanese Patent Provisional Publication No. 2003-142075 describes a lithium secondary battery comprising a positive electrode composition layer having a density of 3.3 to 3.7 g/cm3 provided on aluminum foil, and a negative electrode composition layer having a density of 1.4 to 1.8 g/cm3 provided on copper foil. It is further reported that the obtained lithium secondary battery has high energy density and high safety, and can be preserved at an elevated temperature.